Ummm, is there some reason why an HTTP/1.1 user agent cannot tell
for itself whether or not a message is stale? After all, the
same information that made the proxy decide to add the Warning
header is still present in the cached message, and that information
can be interpreted by the HTTP/1.1 user agent just as easily as the
HTTP/1.1 proxy.
BTW, that begs the question of why the client is being warned about
something that should be obvious from the Date, Age, and Expires/max-age?
As a separate issue, Warning is one of the headers that should be
listed as MUST be sent in a 304 response, with the lack of such a header
meaning remove any existing Warning messages from the cached entity.
That would settle the problem entirely, I think.
.....Roy